What causes my teeth to be sensitive to heat?

Only heat?: Sensitivity to heat is sometimes an indication that a nerve is dead or dying. Being sensitive to hot and cold is very common and can be due to gum recession/exposed root structure, overly aggressive brushing, decay, leaking restorations. But since you said heat only, does it linger for a period of time? In any case, have your dentist check the area to see that there isn't a problem brewing.

Exam needed: It could be from dental caries (tooth decay), broken or defective fillings, nerve inflammation, dental erosions, abrasions or abfractions, gum recession, or perhaps just very sensitive teeth. See your dentist who can give you a thorough dental exam and determine the cause(s).